I work specifically for a consulting firm that has "Flex" hours, which means over the course of two weeks you just need to log 80 hours of time, you could do 8 days of 10 hours, as long as you aren't leaving anyone out to dry. We also have PTO, which is vacation and sick leave combined, making it very reasonable to just bail on a friday if you need to. My question is do you ever feel guilty taking your justified and planned days off? I always have the twinge of guilt that I am a miserable employee and that my peers think I am a slacker, even though I am regularly the first one in and the last one out.

So many things to say about this one, having been there and done that. Let me sum it up tthis way.... When your at work do your best. Do more than is expected and make yourself proud of the job you have done. That is your obligation to your job. When its time to go home get the heck outta there. Go live your life even if it is just taking care of the pets. Think of it as your obligation to yourself.
Are you willing to work extra if asked? The answer should be yes. Whenever it doesnt cramp your after work life too frequently. Some places will take extra every day. selfish places.
Having kids that needed to be picked up from school made leaving the job a bit easier each day. Everyone knew what my drop dead time to leave was and when it hit, I politely packed up and went home. The job was great, but not more important then my kids and if I had to use the excuse that I was working instead of making sure they were not the last children picked up each day, that would slowly killl me inside.
Last bit of lesson here .... Once knew an executive who declared she was married to here job first and her husband second. (Hubby was present when she said this at a business dinner). A year later the job fired her. Then her husband left her. Now she has nothing. Not even children or a pet because those would have gotten in the way of her job. Its a JOB people. It can turn you lose in an instant. Give to the job what you agreed upon and then go live your life.
Steppin off my soap box now. Be well everyone.

In my experience this feeling only happens when I work for a team that isn't healthy. If I work for, or with, people that scramble each year to cash in vacation days because they are 'too busy' to take them throughout the year, then yeah, I feel unreasonably guilty when I take my allotted days. Of course, all I have to do is sit back and actually observe them 'work' - all of those wasted hours micromanaging, or gossiping. Seriously, I've never worked with a 'workaholic' that didn't waste a shit load of time fretting over everyone else's business.

The only other time I feel guilty is if I'm faking or lying, which hasn't happened in decades because, feeling like a douchebag all day isn't worth a day off.

YES, INSANELY. It's so weird. I have 3 vacation days and 10 days off the holiday and one day I was even working from home with my voip to answer customers and emails... AND I STILL FEEL GUILTY. What the hell. I also love my job though, so it helps.

I decided early on that I'd work 8.5 hr days 4 days a week, and then leave at 2:30 on fridays. I told my boss this and he was ok with it, yet every damn friday I feel bad about leaving.

Same thing when I leave at 5 (which is when I get off) . There's this guy there who seems to make a point of mentioning how he was at work all day on saturday (which is not a work day for any of us) , or how he works 70 hours a week, etc. and he happens to sit right across from me. So whenever I leave at 5, I just feel like he's judging me for not staying late every night like he does.

Fuck you dude. I may not have a life, but I'd still rather be at home than at work.

If I feel satisfied that I've done good and hard work, and not slacked off, then I definitely don't feel guilty taking time off. On the other hand, if I've slacked off and then take time off, I just feel stupid. (And probably guilty too.)

It might be that you feel the company is very good and generous to you and maybe that you should repay them by working as much as possible. They do give you time off for a reason - you need it. This past week has been the first week of vacation I've taken for the entire year (even though I've been supposed to take two more weeks earlier in the year), but that was due to schedule. I'm definitely taking my two weeks and more within the next couple months. But this week has been so amazingly restorative.